This invention relates to a dual parison extrusion head and in particular one useful for production of multi-layer plastic parisons.
The use of multiple layer plastic containers for various applications is increasing rapidly. Product packaging for food, cosmetics, chemicals, etc. can often be improved by the use of plastic containers having multiple resin layers exhibiting different material characteristics. For example, catsup containers typically have six layers. From the exterior surface radially inward, these layers include: (1) a polypropylene layer for structural strength and hot fill capability; (2) an adhesive layer; (3) an oxygen barrier layer; (4) a second adhesive layer; (5) a layer of reground plastic resin; and (6) a second polypropylene layer. To produce a multi-layered blow molded plastic container, it is necessary to form a plastic parison having layers of the various plastic resins.
To improve the efficiency of the production process of plastic containers, it is often advantageous to utilize a dual parison extrusion head with resin from a single extruder being used to form two parisons. A dual parison extrusion head divides the flow of resin from the extruder in two, to form the two parisons. By forming two parisons instead of one, the productivity of the process can almost be doubled with little machinery or operating cost increase. In the production of a multi-layered parison, a separate extruder is required for each resin.
It is desireable to minimize the distance between the two parisons in a dual parison extrusion head. By minimizing the parison centerline distance, the size of other machine components can be decreased, such as the molds. With smaller molds, the distance between the various positions the molds must travel through in the production process can be reduced, enabling a possible reduction in the overall process time. Minimum parsion center distance is also desireable when retrofitting a single parison machine with a dual parison head. In such a case, the size of the extrusion head must be minimized to avoid other machinery and reduce the alteration of the machine. It is an object of this invention therefore to minimize the parison center distance in a dual parison extrusion head.
With a dual parison extrusion head, the resin flow from the screw extruders is divided to feed two cylindrical passages to form two parisons. The resin flow to each of the cylindrical passages must be balanced in terms of equal resin flow rate so as to produce parisons and, ultimately plastic containers, which are equal in weight. It is another objective of the present invention therefore, to provide a dual parison head with balanced resin flow rates to each parison to produce containers of equal weight.
In an extrusion head, it is desired to provide resin flow passages which are streamlined to avoid or minimize stagnation points where the resin flow is blocked. Resin can remain at a stagnation for an excessive length of time such that the resin is overheated and begins to degrade. The degraded resin can result in noticeable scorched resin in the molded container. Valves, such as flow plugs used to adjust the resin flow, form restrictions in the flow passage which can result in stagnation points. Accordingly, it is another objective of the present invention to streamline the resin flow passages by minimizing the use of flow plugs to reduce or eliminate stagnation points.
In the formation of a plastic parison, the resin is directed into a vertical cylindrical passage in the extrusion head to form a tubular body of plastic resin. The parison is formed as this plastic body is extruded from the cylindrical passage through an opening at the lower end of the extrusion head. To form the tubular body of resin, the resin typically flows into the cylindrical passage at a single location on the periphery of the cylindrical passage. The resin divides into two streams flowing circumferentially in opposite directions through the cylindrical passage. When these two streams flow into each other, a weld line is formed in the tubular body.
It is often desirable to position this weld line such that it coincides with one of the parting lines of the mold. This results in a finished container having only one line appearing on its exterior surface. Placing the weld line on the parting line is desirable for aesthetics. If the weld line is not hidden on the parting line, the container may be considered to be defective or of low quality by consumers. This is especially true for transparent multi-layer containers in which the weld line from each layer may be visible. Therefore, in the production of a transparent multi-layer container, it is particularly desirable to position the weld line of each layer on the mold parting line. It is a further objective of this invention therefore, to provide a dual parison extrusion head in which the weld line in the parison coincides with one of the parting lines in the associated molds.
The location of the parison weld lines is primarily determined by the location, in the extrusion head, where the resin flow is divided in two to form a tubular resin body. To provide resin conduits in the extrusion head which place the weld line on the mold parting line, while attempting to meet the other objectives of this invention such as, minimizing the parison center line distance, balancing the resin flow rate to each parison and streamlining the flow passages to avoid stagnation is a more difficult task than attempting to achieve each objective individually.
The extrusion head of the present invention has been designed to provide a natural balance of resin flow to the two parisons. With a natural balance, the resin flow to each parison is equal with a minimal need for valving so that the flow passages are streamlined. Natural balance is achieved by providing indentical resin flow conduits to each parison and by providing identical temperature profiles to the resin flow conduits. This is accomplished by making the extrusion head symmetrical about a center plane between the two parison forming cylindrical passages. With mirror-image symmetry, the flow passages will be identical as will be the thermal profiles.
Natural balance is further achieved by designing the resin flow passage such that the pressure drop over the conduits is large. By having a large pressure drop, variations in the resin pressure from one side of the head relative to the other will have little effect on the relative resin flow rates.
The dual parison extrusion head of the present invention includes two cylindrical mandrels in a parallel spaced relation to one another, each having a generally upright longitudinal axis. A die body surrounds the two mandrels and is spaced from each mandrel to form a cylindrical passage between each mandrel and the die body. Each of the cylindrical passages has an open lower end forming an annular outlet orifice at the bottom of the extrusion head.
A series of conduits are provided in the extrusion head for supplying each plastic resin to the cylindrical passages. For each resin used in the multiple layer parison, there is a resin inlet, a pair of resin feed tubes, a pair of annular chambers and a pair of inverted frusto-conical shaped conduits to direct the resin into the cylindrical passages. A separate series of conduits are provided for each resin layer. Each series of conduits are axially spaced from one another.
The resin inlets are positioned along a vertical plane bisecting the extrusion head between the two mandrels. At each inlet, the resin flow stream is divided into the two feed tubes, one tube is directed toward each of the cylindrical passages. The feed tubes direct the resin to the annular chambers which are concentric about the cylindrical passages. The feed tubes join the annular chambers at chamber entrance portions which are located on a vertical bisecting plane containing the longitudinal axes of the two cylindrical passages.
The annular chambers are spaced axially below the chamber entrance portions such that the resin flows substantially downwardly from the feed tubes into the annular chambers. By initiating downward flow of the resin before it flows into the cylindrical passages, the parison center distance can be reduced. From the annular chambers, the resin flows through the inverted frusto-conical conduits into each cylindrical passage to form a downwardly flowing tubular plastic body. These plastic bodies flow through the annular outlet orifices to form tubular parisons below the extrusion head.
The plastic resin from the axially uppermost series of resin conduits flows into the cylindrical passage between the mandrel and the die body. The next adjacent lower series of resin conduits directs resin between the first resin and the die body. Each adjacent lower series of resin conduits directs resin between the outer resin layer and the die body, so as to form a multi-layer plastic resin body which is extruded through the annular outlet orifice to form a multi-layered plastic parison.
Each series of conduits are symmetrical about the vertical plane bisecting the head between the two cylindrical passages. This provides flow and thermal symmetry to the resin to produce a balanced resin flow to minimize the need for valving in the extrusion head which in turn minimizes stagnation and resin degradation.
As the resin flows into the annular chambers, the resin flow is divided into two streams, which flow around the annular chambers in circumferentially opposite directions. The two streams join together forming a weld line diametrically opposite the annular chamber entrance portions. The blow mold cavity is oriented below the extrusion head such that the parting line between the mold halves coincides with the weld line .